• 천문학회지
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• 제2권1호
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• pp.1-9
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• 1969

On the instantaneous tidal relaxation approximation, formulae are derived for the ellipticities and virial theorem of a slightly flattened homogeneous rotating cluster (the largest axis of the cluster is directed towards the Galactic center), in terms of the Galactic tidal force and the characteristic intrinsic plus orbital angular velocity. The expression for a purely tidally-determined ellipticity is identical to that for an incompressible fluid body of uniform density. Orbital motion generally contributes significantly to the shape of the cluster. The virial theorem is identical to that for an isolated cluster except that the gravitational potential energy is multiplied by (1-${\chi}$), where ${\chi}$ is a positive tidal correction term. To obtain the actual mass of a cluster, the virial theorem mass based on an isolated cluster should be multiplied by the factor 1/(1-${\chi}$). The formulae are applied to open star clusters, the globular cluster ${\omega}$ Centauri, and dwarf elliptical galaxies in the Local Group.

• 한국전기전자재료학회논문지
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• 제32권1호
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• pp.58-63
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• 2019

Lithium-ion batteries used for IT, automobiles, and industrial energy-storage devices have battery management systems (BMS) to protect the battery from abnormal voltage, current, and temperature environments, as well as safety devices like, current interruption device (CID), fuse, and vent to obtain positive temperature coefficient (PTC). Nonetheless, there are harmful to human health and property and damage the brand image of the manufacturer because of smoke, fire, and explosion of lithium battery packs. In this paper, we propose a systematic protection algorithm combining battery temperature, over-current, and interconnection between protection elements to prevent copper deposition, internal short circuit, and separator shrinkage due to frequent and instantaneous over-current discharges. The parameters of the proposed algorithm are suggested to utilize the experimental data in consideration of battery pack operating conditions and malicious conditions.

• 전력전자학회:학술대회논문집
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• 전력전자학회 2019년도 전력전자학술대회
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• pp.558-560
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• 2019

범용 인버터는 계통 전원의 전압 강하 또는 정전 발생 시, 정류기의 직류단 전압이 감소하여 인버터 고장(fault)이 발생한다. 보호 동작으로서 인버터는 일시적으로 전원 공급이 이루어 지지 않는 상태에서 제어 기능 및 관리가 이루어 질 수 있도록 Ride-Through 운전을 수행한다. 고장 발생 없이 제어 시스템 상태를 유지하여 전원 복구 후, 빠른 재기동이 가능하다. 본 논문에서 V/f 제어를 사용하는 범용 인버터를 고려한 Ride-Through 방법으로 KEB(Kinetic Energy Backup) 운전을 제안한다. KEB 운전은 순시 정전에 의한 인버터 고장 방지를 위해 요구되는 에너지를 계산하고 슬립 주파수를 조절하여 회생 운전을 통해 인버터의 직류단 전압을 일정하게 유지한다. 제안한 방법은 실험을 통해 유효성을 검증한다.

• Steel and Composite Structures
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• 제33권2호
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• pp.245-259
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• 2019

The present study aims to investigate the ultimate strength and geometric nonlinear behavior of composite plates containing initial imperfection subjected to combined end-shortening strain and lateral pressure loading by using a semi-analytical method. In this study, the first order shear deformation plate theory is considered with the assumption of large deflections. Regarding in-plane boundary conditions, two adjacent edges of the laminates are completely held while the two others can move straightly. The formulations are based on the concept of the principle of minimum potential energy and Newton-Raphson technique is employed to solve the nonlinear set of algebraic equations. In addition, Hashin failure criteria are selected to predict the failures. Further, two distinct models are assumed to reduce the mechanical properties of the failure location, complete ply degradation model, and ply region degradation model. Degrading the material properties is assumed to be instantaneous. Finally, laminates having a wide range of thicknesses and initial geometric imperfections with different intensities of pressure load are analyzed and discuss how the ultimate strength of the plates changes.

• Ocean Systems Engineering
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• 제14권2호
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• pp.141-156
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• 2024

The turbine system converts the kinetic energy of water flow to electricity by rotating the rotor in a restricted waterway between the seabed and free surface. A turbine system's immersion depth and rotation direction are significantly critical in the turbine's performance along with the shape of the rotor. This study has investigated the hydrodynamic performance of the Savonius hydrokinetic turbine (SHT) according to the immersion depth and rotation direction using computational fluid dynamics (CFD) simulations. The instantaneous torque, torque coefficient, and power coefficients are calculated for the immersion ratios Z/D ranging [0.25, 3.0] and both clockwise (CW) and counterclockwise (CCW) rotations. A flow visualization around the rotor is shown to clarify the correlation between the turbine's performance and the flow field. The CFD simulations show that the CCW rotation produces a higher power at shallow immersion, while the CW rotation performs better at deeper immersion. The immersion ratio should be greater than the minimum of Z/D=1.0 to obtain the maximum power production regardless of the rotation direction.

• Korean Chemical Engineering Research
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• 제53권5호
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• pp.540-544
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• 2015

나노입자를 기판 위에 직접 부착시키는 방법인 전착(electrodeposition)을 이용하여 저가의 그라파이트(graphite) 기판 위에 백금 나노입자를 직접 부착시킬 수 있는 전착 욕(bath)을 개발하였고, 백금 나노입자 전착반응의 전기화학적인 특성을 분석하였다. 백금 나노입자 전착의 분극 거동 분석을 통하여 반응메카니즘을 파악하였고, 순환전위측정(cyclic voltammetry)을 통하여 백금 나노입자 전착에서는 물질전달이 속도결정단계임을 확인하였다. 또한 시간대전류법(chronoamperometry)으로 분석한 백금 나노입자 전착의 전류밀도 변화 양상은 백금 나노입자의 결정핵 생성 메카니즘이 instantaneous로 판명되었다. 그라파이트는 다른 탄소계열 기판에 비하여 매우 저가이기 때문에 그라파이트 기판 위에 백금 나노입자를 직접 부착시키는 기술은 산업적으로 유용할 것으로 기대한다.

• 방송공학회논문지
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• 제18권6호
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• pp.884-894
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• 2013

최근 지구 온난화에 따른 이산화탄소 배출량 감소 및 에너지 절약을 위하여 방송통신 기술 역시 에너지를 절약을 위한 연구가 시작되고 있다. 특히 방송통신 시스템의 전체 소비 에너지는 기지국에서 87.5%이상 차지하고 있지만 물리계층에서 에너지 효율성에 관한 연구는 전송용량을 총 전력 단일 상수로 나누어 분석하기 때문에 실제적인 에너지 특성을 분석하는데 문제점이 많다. 본 논문은 이와 같은 문제점을 해결하기 위하여 실제 증폭기에 사용되는 트랜지스터의 특성과 신호 모델에 관하여 간략히 소개한다. 이는 에너지 효율적인 방송통신 시스템을 설계하는데 이용 가능한 비선형 증폭기의 신호모델 그리고 증폭기의 특성을 고려한 기지국의 총 전력 사용량까지 포함하여 효율적인 에너지 효율을 계산하는 데 중요한 이해를 제시한다.

• 한국지열·수열에너지학회논문집
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• 제17권1호
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• pp.14-22
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• 2021

Drilling equipment is an essential part used in various fields such as construction, mining, etc., and it has drawn increasing attention in recent years. The drilling method is generally divided into three types. There are a top hammer method that strikes on the ground, a DTH (Down-The-Hole) method that directly strikes a bit in an underground area, and a rotary method that drills by using rotational force. Among them, the DTH method is most commonly used because it enables efficient drilling compared to other drilling methods. In the conventional DTH hammer, the valve between the piston and the bit is opened and closed using a face to face method. In order to improve the power of the DTH hammer, a DTH hammer with foot valve which is capable of instantaneous opening and closing is used in the drilling field. In this study, we designed a lab-scale DTH hammer with the foot valve, and manufactured an evaluation device for the experiment of the DTH hammer. In addition, we analyzed the performance of the DTH hammer adopted with foot valve according to the pressure range of 3-10 bar. As a result, the internal pressure distribution in the DTH hammer was experimentally analyzed, and then, the movement of the piston according to the pressure was predicted. We believe that this study provides the useful results to explain the performance characteristics of the DTH hammer with the foot valve.

• 한국포장학회지
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• 제29권3호
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• pp.223-228
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• 2023

Sustainable energy supplies without the recharging and replacement of the charge storage device have become increasingly important. Among various energy harvesters, the triboelectric nanogenerator (TENG) has attracted considerable attention due to its high instantaneous output power, broad selection of available materials, eco-friendly and inexpensive fabrication process, and various working modes customized for target applications. In this study, the amount of voltage and current generated was measured by applying the PSD profile random vibration test of the electronic vibration tester and ISTA 3A according to the time of Anodized Aluminum Oxide (AAO) pore widening of the manufactured TENG device Teflon and AAO. The discharge and charging tests of the integrated module during the random simulated transport environment and the recognition distance of RFID were measured while agricultural products (onion) were loaded into the returnable folding plastic box. As a result, it was found that AAO alumina etching processing time to maximize TENG performance was optimal at 31 min in terms of voltage and current generation, and the integrated module applied with the TENG module showed a charging effect even during the continuous use of RFID, so the voltage was kept constant without discharge. In addition, the RFID recognition distance of the integrated module was measured as a maximum of 1.4 m. Therefore, it was found that the surface condition of AAO, a TENG element, has a great influence on the power generation of the integrated module, and due to the characteristics of TENG, the power generation increases as the surface dries, so it is judged that the power generation can be increased if the surface drying treatment (ozone treatment, etc.) of AAO is applied in the future.

• Journal of Welding and Joining
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• 제11권3호
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• pp.34-47
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• 1993

용접에서 발생하는 열응력 및 잔류응력을 해석하기 위한 유한요소용 모델을 개발하였다. 여러 가 지 변수의 연구를 통하여 Ramp heat input function과 Lumped모델을 제시하였다. 용접부에 열입 력을 점차적으로 주기 위하여 Ramp heat input을 이용하였으며 Ramp input을 통하여 이차원 모 델에서의 이동열원의 영향을 고려하였고 실험치와 비교에서 최적 ramp시간을 결정하였다. 다층용 접에서는 용접 pass 에 비례하여 계산시간이 증가한다. 따라서 후판용접의 잔류응력계산에는 막 대한 계산시간이 필요하며 이를 줄이기 위하여 Lumped 모델을 개발하였다. 이 Lumped모델에서 는 각 용접층에 들어있는 용접 pass들을 하나의 lumped pass으로 이용하였으며 각 pass를 따로 계산한 모델 및 시험치와의 비교를 통하여 최적 lumped technique을 제시하였다. *****Finite element models were developed for thermal and residual stress analysis for the specific welding problems. They were used to evaluate the effectiveness of the various welding heat input models, such as ramp heat input function and lumped pass models. Through the parametric studies, thermal-mechanical modeling sensitivity to the ramp function and lumping techniques was determined by comparing the predicted results with experimental data. The kinetics for residual stress formation during welding can be developed by iteration of various proposed mechanisms in the parametric study. A ramp heat input function was developed to gradually apply the heat flux with variable amplitude to the model. This model was used to avoid numerical convergence problems due to an instantaneous increase in temperature near the fusion zone. Additionally, it enables the model to include the effect of a moving arc in a two-dimensional plane. The ramp function takes into account the variation in the out of plane energy flow in a 2-D model as the arc approaches, travels across, and departs from each plane under investigation. A lumped pass model was developed to reduce the computation cost in the analysis of multipass welds. Several weld passes were assumed as one lumped pass in this model. Recommendations were provided about ramp lumping techniques and the optimum number of weld passes that can be combined into a single thermal input.